I agree with Rat that they should probably spawn in zombie raids as well, probably also acting as a zombie jockey. I would like to also suggest the creation of a drowned horse, which, on top of just looking cool, would be like a halfway stage between skeleton and zombie horses. I'd suggest that it would have half the health of the skeleton horse and, like the drowned, would be able to properly swim instead of just sinking as skeleton and zombie horses do. As for natural spawning I feel like perhaps not every undead should have their own mount, but they could be found around/in a new sunken stable structure. Also obviously if a zombie horse drowns. I hope you lovely folks at the Mojang team take these ideas into consideration.